1. Field of the Invention
This invention relates to a differential amplifier circuit.
2. Description of the Background Art
FIG. 5 shows a single output type mutual-conductance differential amplifier circuit (hereafter referred to as a gm amplifier circuit). In FIG. 5, the emitters of transistors Q21 and Q22 are connected to a constant-current circuit I21, and their collectors are connected to a current mirror circuit constituted by transistors Q23 and Q24 and resistances R21 and R22. IN0 and IN1 are input terminals of the gm amplifier circuit, and are connected to the bases of the transistors Q22 and Q21 respectively. On the other hand, OUTA is a current output terminal, and is connected to the collector of the transistor Q22. A capacitor C21 is a load capacity of the gm amplifier circuit, and converts a current signal from the OUTA into a voltage signal. A buffer B21, and outputs at a low impedance a voltage signal from C21 through an output terminal OUT to the outside. FIG. 6 is a symbolic representation of the circuit of FIG. 5.
FIG. 7 is an example of an active filter circuit constituted by the gm amplifier circuit as shown in FIG. 6 (or FIG. 5), and this circuit operates as a primary low-pass filter.
An active filter circuit using a conventional single-output gm amplifier circuit, such as this, tends to allow noise to be mixed in the signal when the supply voltage fluctuates. That is to say, the supply voltage fluctuation removal ratio is insufficient, which is a problem.